Titanium sintered body and ornament

ABSTRACT

A titanium sintered body contains an α-phase and a β-phase as crystal structures, wherein the average grain size of the α-phase in a cross section is 3 μm or more and 30 μm or less, and an area ratio occupied by the α-phase in a cross section is 70% or more and 99.8% or less. In the titanium sintered body, it is preferred that the average aspect ratio of the α-phase in a cross section is 1 or more and 3 or less. It is also preferred that the titanium sintered body contains titanium as a main component, and also contains an α-phase stabilizing element and a β-phase stabilizing element.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Japanese Patent Application Nos.2015-176071 filed Sep. 7, 2015 and 2016-107641 filed May 30, 2016. Theentire disclosures of Japanese Patent Application Nos. 2015-176071 and2016-107641 are hereby incorporated herein by reference.

BACKGROUND

1. Technical Field

The present invention relates to a titanium sintered body and anornament.

2. Related Art

A titanium alloy has excellent mechanical strength and corrosionresistance, and therefore has been used in the field of aircraft, spacedevelopment, chemical plants, and the like. Further, recently, byutilizing the characteristics such as biocompatibility and a low Young'smodulus of a titanium alloy, a titanium alloy has begun to be applied toexterior components of watches, ornaments such as glasses frames,sporting goods such as golf clubs, springs, and the like.

Further, in the application of a titanium alloy in this manner, by usinga powder metallurgy method, a titanium sintered body having a shapeclose to the final shape can be easily produced. Therefore, secondaryprocessing can be omitted or a processing amount can be reduced, andthus, components can be efficiently produced.

However, a titanium sintered body produced by a powder metallurgy methodis likely to reflect the properties of a starting material powder, andit is difficult to increase the surface smoothness. Due to this, thespecularity of the titanium sintered body is easy to decrease, which cancause a problem of appearance.

Therefore, an attempt to improve the specularity of a titanium sinteredbody produced by a powder metallurgy method has been proposed.

For example, JP-A-8-92674 (PTL 1) discloses a titanium alloy forornaments obtained by powder compacting a mixed powder containing aniron powder in an amount of 0.1 to 1.0% by weight and a molybdenumpowder in an amount of 0.1 to 4.0% by weight with the remainderconsisting of a titanium powder, followed by sintering at 1200 to 1350°C. Further, PTL 1 describes that the obtained titanium alloy contains anα+β two-phase structure, and specularity required for an externalcomponent of watches or the like is obtained.

However, the titanium alloy disclosed in PTL 1 contains iron in additionto titanium, and therefore has poor weather resistance. Due to this, inthe case where the titanium alloy is exposed to a harsh environment fora long period of time, deterioration occurs on the surface, resulting inlowering the specularity.

SUMMARY

An advantage of some aspects of the invention is to provide a titaniumsintered body and an ornament capable of maintaining good specularityfor a long period of time.

A titanium sintered body according to an aspect of the inventioncontains an α-phase and a β-phase as crystal structures, wherein theaverage grain size of the α-phase in a cross section is 3 μm or more and30 μm or less, and an area ratio occupied by the α-phase in a crosssection is 70% or more and 99.8% or less.

With this configuration, a titanium sintered body capable of maintaininggood specularity for a long period of time is obtained.

In the titanium sintered body according to the aspect of the invention,it is preferred that the average aspect ratio of the α-phase in a crosssection is 1 or more and 3 or less.

With this configuration, anisotropy is less likely to occur in apolishing amount when polishing processing is performed for the titaniumsintered body, and therefore, unevenness is less likely to occur on thepolished surface. Due to this, the smoothness of the polished surfacecan be further increased, and thus, a titanium sintered body havingparticularly excellent specularity is obtained.

In the titanium sintered body according to the aspect of the invention,it is preferred that in an X-ray diffraction spectrum obtained by X-raydiffractometry, the value of a peak reflection intensity by the planeorientation (110) of the β-phase is 3% or more and 60% or less of thevalue of a peak reflection intensity by the plane orientation (100) ofthe α-phase.

With this configuration, both characteristics of the α-phase andcharacteristics of the β-phase become obvious without being buried. As aresult, a titanium sintered body capable of maintaining high specularityparticularly for a long period of time is obtained.

In the titanium sintered body according to the aspect of the invention,it is preferred that the titanium sintered body contains titanium as amain component, and also contains an α-phase stabilizing element and aβ-phase stabilizing element.

With this configuration, even if the production conditions or useconditions for the titanium sintered body change, since the titaniumsintered body can have both α-phase and β-phase as the crystalstructures, the titanium sintered body has excellent weather resistance.As a result, the titanium sintered body has both characteristicsexhibited by the α-phase and characteristics exhibited by the β-phase,and thus has particularly excellent mechanical properties.

In the titanium sintered body according to the aspect of the invention,it is preferred that the titanium sintered body has a relative densityof 99% or more.

With this configuration, when the surface of the titanium sintered bodyis polished, particularly good specularity is exhibited.

An ornament according to an aspect of the invention includes thetitanium sintered body according the aspect of the invention.

With this configuration, an ornament capable of maintaining goodspecularity for a long period of time, and as a result, capable ofmaintaining excellent aesthetic appearance for a long period of time isobtained.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to the accompanyingdrawings, wherein like numbers reference like elements.

FIG. 1 is an electron microscopic image showing an embodiment of atitanium sintered body according to the invention.

FIG. 2 is a view schematically drawing a part of the electronmicroscopic image shown in FIG. 1.

FIG. 3 is a perspective view showing a watch case to which an embodimentof an ornament according to the invention is applied.

FIG. 4 is a partial cross-sectional perspective view showing a bezel towhich an embodiment of an ornament according to the invention isapplied.

FIG. 5 is an X-ray diffraction spectrum obtained for a titanium sinteredbody of Example 1.

FIG. 6 is an electron microscopic image of a cross section of a titaniumsintered body of Comparative Example 2.

FIG. 7 is an electron microscopic image of a cross section of a titaniumingot material of Reference Example 1.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, a titanium sintered body and an ornament according to theinvention will be described in detail with reference to preferredembodiments shown in the accompanying drawings.

Titanium Sintered Body

First, an embodiment of the titanium sintered body according to theinvention will be described.

FIG. 1 is an electron microscopic image showing an embodiment of thetitanium sintered body according to the invention, and FIG. 2 is a viewschematically drawing a part of the electron microscopic image shown inFIG. 1. Incidentally, FIG. 1 is obtained by taking an image of a cutsurface of the titanium sintered body, and a dark-colored band extendingto the right and left in the upper end of FIG. 1 is a region outside thetitanium sintered body.

The titanium sintered body according to this embodiment is produced by,for example, a powder metallurgy method. That is, this titanium sinteredbody is formed by sintering titanium alloy powder particles to oneanother.

More specifically, as shown in FIG. 2, a titanium sintered body 1contains an α-phase 2 and a β-phase 3 as crystal structures. Amongthese, the α-phase 2 refers to a region (α-phase titanium) in which thecrystal structure forming the same is mainly a hexagonal closest packed(hcp) structure. On the other hand, the β-phase 3 refers to a region(β-phase titanium) in which the crystal structure forming the same ismainly a body-centered cubic (bcc) structure. In FIG. 1, the α-phase 2appears as a region with a relatively light color, and the β-phase 3appears as a region with a relatively dark color.

The α-phase 2 has relatively low hardness and high ductility, andtherefore contributes to the realization of the titanium sintered body 1having excellent strength and excellent deformation resistanceparticularly at a high temperature. On the other hand, the β-phase 3 hasrelatively high hardness, but is likely to be plastically deformed, andtherefore contributes to the realization of the titanium sintered body 1having excellent toughness as a whole.

It is preferred that most of the cross section of the titanium sinteredbody 1 is occupied by such an α-phase 2 and a β-phase 3. The totaloccupancy ratio (area ratio) of the α-phase 2 and the β-phase 3 is notparticularly limited, but is preferably 95% or more, more preferably 98%or more. In such a titanium sintered body 1, the α-phase 2 and theβ-phase 3 become dominant in characteristics, and therefore, thetitanium sintered body 1 reflects many advantages of titanium.

The total occupancy ratio of the α-phase 2 and the β-phase 3 is obtainedby, for example, observing the cross section of the titanium sinteredbody 1 by an electron microscope or a light microscope anddistinguishing the crystal phases based on the difference in color orthe contrast due to the difference in crystal structure and alsomeasuring the areas.

Examples of crystal structures other than the α-phase 2 and the β-phase3 include a ω-phase and a γ-phase.

The titanium sintered body 1 contains the α-phase 2 and the β-phase 3 asdescribed above, and also the average grain size of the α-phase 2 is 3μm or more and 30 μm or less, and the occupancy ratio (area ratio) ofthe α-phase 2 is 70% or more and 99.8% or less.

In such a titanium sintered body 1, since the α-phase 2 is minute andalso the α-phase 2 is dominant, the strength is high and also theuniformity of polishing processing is high. Due to this, when polishingprocessing is performed for the titanium sintered body 1, unevennesscaused by a difference in the hardness between the α-phase 2 and theβ-phase 3 is less likely to occur, and thus, the smoothness of thepolished surface can be increased. In addition, the α-phase 2 which isdominantly present hardly causes translocation and therefore is hardlydenatured by polishing and also has high corrosion resistance, and thuscontributes to the maintenance of a smooth state immediately afterpolishing for a long period of time. In other words, the wear resistanceis high, and therefore, scratching or the like of the polished surfaceis suppressed, so that the polished surface can be kept good for a longperiod of time. On the other hand, the β-phase 3 whose amount ofpresence is smaller than that of the α-phase 2 is likely to beplastically deformed as described above, and therefore promotes themutual sliding of the grains of the α-phase 2. Due to this, even ifstress is applied during polishing processing, the stress can bealleviated in the β-phase 3. As a result, a problem such as a decreasein smoothness due to residual stress can be prevented from occurring. Inother words, a polishing property enabling favorable polishing isobtained, and therefore, a polished surface having high specularity canbe easily obtained.

When the average grain size of the α-phase 2 is less than the abovelower limit, the grain size of the α-phase 2 is too small, andtherefore, it is difficult to achieve appropriate polishing, and alsothe α-phase 2 having a small grain size is likely to affect thereflection of light, and thus, the specularity of the polished surfacemay be decreased. In addition, the occupancy ratio of the α-phase 2cannot be sufficiently increased, and therefore, the mechanical strengthof the titanium sintered body 1 may not be able to be sufficientlyincreased. On the other hand, when the average grain size of the α-phase2 exceeds the above upper limit, the α-phase 2 is likely to have aneedle shape. When the α-phase 2 has a needle shape, the fatiguestrength of the titanium sintered body 1 is easy to decrease, andtherefore, it becomes difficult to maintain high specularity for a longperiod of time. Further, the wear resistance is decreased, andtherefore, the polished surface is likely to be scratched, and it may bedifficult to keep the polished surface good for a long period of time.In addition, the mechanical strength attributed mainly to the α-phase 2may be decreased.

When the area ratio occupied by the α-phase 2 is lower than the abovelower limit, the area occupied by the β-phase 3 is increased by thatamount. Due to this, the degree of contribution to the reflection oflight by the β-phase 3 is increased, and thus, the specularity of thepolished surface is decreased. On the other hand, when the area ratiooccupied by the α-phase 2 exceeds the above upper limit, the amount ofpresence of the β-phase 3 is decreased by that amount. Due to this, thefunction of the β-phase 3 of alleviating the stress generated betweenthe grains of the α-phase 2 is deteriorated, and therefore, thesmoothness of the polished surface may be decreased due to residualstress.

The average grain size of the α-phase 2 is preferably 5 μm or more and25 μm or less, more preferably 7 μm or more and 20 μm or less.

The average grain size of the α-phase 2 is measured as follows. First,the cross section of the titanium sintered body 1 is observed by anelectron microscope, and 100 or more grains of the α-phase 2 in theobtained observation image are randomly selected. Subsequently, the areaof each grain of the α-phase 2 selected in the observation image iscalculated, and the diameter of a circle having the same area as that ofthis area is obtained. The diameter of the circle obtained in thismanner is regarded as the grain size (equivalent circle diameter) of thegrain of the α-phase 2, and an average for 100 or more grains of theα-phase 2 is obtained. This average is determined as the average grainsize of the α-phase 2.

The area ratio occupied by the α-phase 2 is preferably 75% or more and99% or less, more preferably 80% or more and 98% or less.

The area ratio occupied by the α-phase 2 is measured as follows. First,the cross section of the titanium sintered body 1 is observed by anelectron microscope, and the area of the obtained observation image iscalculated. Subsequently, the total area of the α-phase 2 in theobservation image is obtained. Then, the obtained total area of theα-phase 2 is divided by the area of the observation image. The solutionis the area ratio occupied by the α-phase 2.

On the other hand, in the case where the α-phase 2 is present at an arearatio as described above, the area ratio of the β-phase 3 is smallerthan that. Specifically, the area ratio of the β-phase 3 is preferablyabout 0.2% or more and 30% or less, more preferably about 1% or more and25% or less, further more preferably about 2% or more and 20% or less.The β-phase 3 is likely to be plastically deformed as described above,and therefore promotes the mutual sliding of the grains of the α-phase2. Due to this, in the case where the β-phase 3 is present at a ratiowithin the above range, when the titanium sintered body 1 is polished,the resistance during polishing can be prevented from significantlyincreasing. As a result, the smoothness of the polished surface can befurther increased, and thus, the titanium sintered body 1 having highspecularity and excellent aesthetic appearance can be obtained.

The constituent material of such a titanium sintered body 1 is atitanium simple substance or a titanium-based alloy.

The titanium-based alloy is an alloy containing titanium as a maincomponent, but is an alloy containing, other than titanium (Ti), anelement such as carbon (C), nitrogen (N), oxygen (O), aluminum (Al),vanadium (V), niobium (Nb), zirconium (Zr), tantalum (Ta), molybdenum(Mo), chromium (Cr), manganese (Mn), cobalt (Co), iron (Fe), silicon(Si), gallium (Ga), tin (Sn), barium (Ba), nickel (Ni), or sulfur (S).

Among these, the titanium-based alloy according to this embodimentpreferably contains an α-phase stabilizing element and a β-phasestabilizing element. According to this, even if the productionconditions or use conditions for the titanium sintered body 1 change,since the titanium sintered body 1 can have both α-phase 2 and β-phase 3as the crystal structures, the titanium sintered body 1 has excellentweather resistance. Due to this, the titanium sintered body 1 has bothcharacteristics exhibited by the α-phase 2 and characteristics exhibitedby the β-phase 3, and thus has particularly excellent mechanicalproperties.

Examples of the α-phase stabilizing element include aluminum, gallium,tin, carbon, nitrogen, and oxygen, and these are used alone or two ormore types thereof are used in combination. On the other hand, examplesof the β-phase stabilizing element include molybdenum, niobium,tantalum, vanadium, and iron, and these are used alone or two or moretypes thereof are used in combination.

As a specific composition of the titanium-based alloy, a titanium alloyspecified in JIS H 4600:2012 as type 60, type 60E, type 61, or type 61Fcan be used. Specific examples thereof include Ti-6Al-4V, Ti-6Al-4V ELI,and Ti-3Al-2.5V. Other examples thereof include Ti-6Al-6V-2Sn,Ti-6Al-2Sn-4Zr-2Mo-0.08Si, and Ti-6Al-2Sn-4Zr-6Mo specified in AerospaceMaterial Specifications (AMS). Further, additional examples thereofinclude Ti-5Al-2.5Fe and Ti-6Al-7Nb specified in the specification madeby International Organization for Standardization (ISO), and alsoinclude Ti-13Zr-13Ta, Ti-6Al-2Nb-1Ta, Ti-15Zr-4Nb-4Ta, andTi-5Al-3Mo-4Zr.

In the notation of the above-mentioned alloy composition, the componentsare shown in decreasing order of concentration from left to right, andthe number shown before the element indicates the concentration of theelement in mass %. For example, Ti-6Al-4V shows that the alloy containsAl at 6 mass % and V at 4 mass % with the remainder consisting of Ti andimpurities. The impurities are elements which are inevitably containedor elements which are added intentionally at a predetermined ratio (forexample, the total amount of the impurities is 0.40 mass % or less).

Further, the ranges for main alloy compositions described above are asfollows.

The Ti-6Al-4V alloy contains Al at 5.5 mass % or more and 6.75 mass % orless and V at 3.5 mass % or more and 4.5 mass % or less with theremainder consisting of Ti and impurities. As the impurities, forexample, Fe at 0.4 mass % or less, O at 0.2 mass % or less, N at 0.05mass % or less, H at 0.015 mass % or less, and C at 0.08 mass % or lessare permitted to be contained, respectively. Further, other elements arepermitted to be contained at 0.10 mass % or less independently and 0.40mass % or less in total, respectively.

The Ti-6Al-4V ELI alloy contains Al at 5.5 mass % or more and 6.5 mass %or less and V at 3.5 mass % or more and 4.5 mass % or less with theremainder consisting of Ti and impurities. As the impurities, forexample, Fe at 0.25 mass % or less, O at 0.13 mass % or less, N at 0.03mass % or less, H at 0.0125 mass % or less, and C at 0.08 mass % or lessare permitted to be contained, respectively. Further, other elements arepermitted to be contained at 0.10 mass % or less independently and 0.40mass % or less in total, respectively.

The Ti-3Al-2.5V alloy contains Al at 2.5 mass % or more and 3.5 mass %or less, Vat 1.6 mass % or more and 3.4 mass % or less, S (according toneed) at 0.05 mass % or more and 0.20 mass % or less, and at least oneelement (according to need) selected from La, Ce, Pr, and Nd at 0.05mass % or more and 0.70 mass % or less in total with the remainderconsisting of Ti and impurities. As the impurities, for example, Fe at0.30 mass % or less, O at 0.25 mass % or less, N at 0.05 mass % or less,H at 0.015 mass % or less, and C at 0.10 mass % or less are permitted tobe contained, respectively. Further, other elements are permitted to becontained at 0.40 mass % or less in total.

The Ti-5Al-2.5Fe alloy contains Al at 4.5 mass % or more and 5.5 mass %or less and Fe at 2 mass % or more and 3 mass % or less with theremainder consisting of Ti and impurities. As the impurities, forexample, 0 at 0.2 mass % or less, N at 0.05 mass % or less, H at 0.013mass % or less, and C at 0.08 mass % or less are permitted to becontained, respectively. Further, other elements are permitted to becontained at 0.40 mass % or less in total.

The Ti-6Al-7Nb alloy contains Al at 5.5 mass % or more and 6.5 mass % orless and Nb at 6.5 mass % or more and 7.5 mass % or less with theremainder consisting of Ti and impurities. As the impurities, forexample, Ta at 0.50 mass % or less, Fe at 0.25 mass % or less, O at 0.20mass % or less, N at 0.05 mass % or less, H at 0.009 mass % or less, andC at 0.08 mass % or less are permitted to be contained, respectively.Further, other elements are permitted to be contained at 0.40 mass % orless in total. The Ti-6Al-7Nb alloy has particularly low cytotoxicity ascompared with other alloy types, and therefore is particularly usefulwhen the titanium sintered body 1 is used for biocompatible purposes.

The components contained in the titanium sintered body 1 can be analyzedby, for example, a method in accordance with Titanium—ICP atomicemission spectrometry specified in JIS H 1632-1 (2014) to JIS H 1632-3(2014).

The titanium sintered body 1 may also contain particles containingtitanium oxide as a main component (hereinafter simply referred to as“titanium oxide particles”). It is considered that the titanium oxideparticles share the stress applied to titanium metal serving as thematrix by being dispersed in the titanium sintered body 1. Due to this,by including the titanium oxide particles, the mechanical strength ofthe entire titanium sintered body 1 is improved. Further, since titaniumoxide is harder than titanium metal, by dispersing the titanium oxideparticles, the wear resistance of the titanium sintered body 1 can befurther increased. Due to this, scratching or the like of the polishedsurface is suppressed, and therefore, the polished surface can be keptgood for a long period of time.

The “particles containing titanium oxide as a main component” refers to,for example, particles analyzed such that an element contained in thelargest amount is either one of titanium and oxygen, and an elementcontained in the second largest is the other when a component analysisof the particles of interest is performed by an X-ray fluorescenceanalysis or an electron microprobe analyzer.

The average particle diameter of the titanium oxide particles is notparticularly limited, but is preferably 0.5 μm or more and 20 μm orless, more preferably 1 μm or more and 15 μm or less, further morepreferably 2 μm or more and 10 μm or less. When the average particlediameter of the titanium oxide particles is within the above range, thewear resistance can be increased without largely deteriorating themechanical properties such as toughness and tensile strength of thetitanium sintered body 1. That is, when the average particle diameter ofthe titanium oxide particles is less than the above lower limit, theeffect of sharing the stress of the titanium oxide particles may bedecreased depending on the content of the titanium oxide particles.Further, when the average particle diameter of the titanium oxideparticles exceeds the above upper limit, the titanium oxide particle mayserve as a starting point of a crack to decrease the mechanical strengthdepending on the content of the titanium oxide particles.

The crystal structure of the titanium oxide particle may be any of arutile type, an anatase type, and a brookite type, and may be a mixtureof a plurality of types.

The average particle diameter of the titanium oxide particles ismeasured as follows. First, the cross section of the titanium sinteredbody 1 is observed by an electron microscope, and 100 or more titaniumoxide particles in the obtained observation image are randomly selected.At this time, whether a particle is the titanium oxide particle or notcan be specified by the contrast of the image and an area analysis ofoxygen or the like. Subsequently, the area of each titanium oxideparticle selected in the observation image is calculated, and thediameter of a circle having the same area as that of this area isobtained. The diameter of the circle obtained in this manner is regardedas the particle diameter (equivalent circle diameter) of the titaniumoxide particle, and an average for 100 or more titanium oxide particlesis obtained. This average is determined as the average particle diameterof the titanium oxide particles.

The shape of the α-phase 2 according to this embodiment is preferablynot a needle shape, but an isotropic shape or a shape equivalentthereto. When the α-phase 2 has such a shape, the decrease in thefatigue strength of the titanium sintered body 1 can be suppressed asdescribed above. As a result, the titanium sintered body 1 capable ofmaintaining high specularity for a long period of time is obtained.

Specifically, in the cross section of the titanium sintered body 1, theaverage aspect ratio of the α-phase 2 is preferably 1 or more and 3 orless, more preferably 1 or more and 2.5 or less. When the average aspectratio of the α-phase 2 is within the above range, the decrease in thefatigue strength and the hardness of the titanium sintered body 1 issuppressed. Due to this, the titanium sintered body 1 which is useful asa structural component is obtained. Further, by adjusting the averageaspect ratio within the above range, anisotropy is less likely to occurin a polishing amount when polishing processing is performed for thetitanium sintered body 1, and therefore, unevenness is less likely tooccur on the polished surface. As a result, the smoothness of thepolished surface can be further increased, and thus, the titaniumsintered body 1 having particularly excellent specularity is obtained.In other words, when anisotropy is likely to occur in a polishingamount, anisotropy also occurs in light reflection, and thus, thespecularity or the aesthetic properties may be decreased.

The average aspect ratio of the α-phase 2 is measured as follows. First,the cross section of the titanium sintered body 1 is observed by anelectron microscope, and 100 or more grains of the α-phase 2 in theobtained observation image are randomly selected. Subsequently, themajor axis of the grain of the α-phase 2 selected in the observationimage is specified, and further, the longest axis in the directionorthogonal to this major axis is specified as the minor axis. Then, theratio of the major axis to the minor axis is calculated as the aspectratio. Then, the aspect ratios of 100 or more grains of the α-phase 2 isaveraged, and the resulting value is determined as the average aspectratio.

In the titanium sintered body 1 according to this embodiment, theα-phase 2 has a relatively uniform grain size. Due to this, not onlybecause of having an isotropic shape or a shape equivalent thereto, butalso having a uniform grain size, the fatigue strength of the titaniumsintered body 1 is increased, and also the specularity can be kept highfor along period of time.

When the measurement result of the grain size of the α-phase 2 isplotted in a plot area in which the horizontal axis represents the grainsize of the α-phase 2 and the vertical axis represents the number ofgrains of the α-phase 2 corresponding to the grain size, a grain sizedistribution of the α-phase 2 is obtained. In this grain sizedistribution, the grain size when the cumulative number of grains fromthe small grain size side reaches 16% of the total is represented byD16, and the grain size when the cumulative number of grains from thesmall grain size side reaches 84% of the total is represented by D84. Atthis time, the standard deviation SD of the grain size distribution isobtained according to the following formula.

SD=(D84−D16)/2

The standard deviation SD obtained in this manner serves as the index ofthe distribution width of the grain size distribution. In the titaniumsintered body 1 according to this embodiment, the standard deviation SDof the grain size distribution of the α-phase 2 is preferably 5 or less,more preferably 3 or less, further more preferably 2 or less. In thetitanium sintered body 1 in which the standard deviation SD of the grainsize distribution of the α-phase 2 is within the above range, the grainsize distribution is sufficiently narrow, and also the grain size of theα-phase 2 is sufficiently uniform. Such a titanium sintered body 1 hasparticularly high fatigue strength, and can maintain high specularityfor a long period of time.

Further, an X-ray diffraction spectrum obtained by subjecting thetitanium sintered body 1 to a crystal structure analysis by X-raydiffractometry includes a peak reflection intensity derived from theα-phase and a peak reflection intensity derived from the β-phase.

Here, it is preferred that the obtained X-ray diffraction spectrumparticularly includes a peak reflection intensity by the planeorientation (100) of the α-phase titanium and a peak reflectionintensity by the plane orientation (110) of the β-phase titanium. Inaddition, the value of the peak reflection intensity (the value of thepeak top) by the plane orientation (110) of the β-phase titanium ispreferably 3% or more and 60% or less, more preferably 5% or more and50% or less, further more preferably 10% or more and 40% or less of thevalue of the peak reflection intensity (the value of the peak top) bythe plane orientation (100) of the α-phase titanium. According to this,both characteristics of the α-phase 2 and characteristics of the β-phase3 described above become obvious without being buried. As a result, thetitanium sintered body 1 capable of maintaining high specularityparticularly for a long period of time is obtained.

The peak reflection intensity by the plane orientation (100) of theα-phase titanium is located at 2θ of about 35.3°. On the other hand, thepeak reflection intensity by the plane orientation (110) of the β-phasetitanium is located at 2θ of about 39.5°.

As the X-ray source of the X-ray diffractometer, Cu-Kα radiation isused, and the tube voltage is set to 30 kV, and the tube current is setto 20 mA.

Further, the titanium sintered body 1 has a relative density ofpreferably 99% or more, more preferably 99.5% or more. By setting therelative density of the titanium sintered body 1 within the above range,the titanium sintered body 1 having particularly good specularity whenpolishing the surface is obtained. That is, when the titanium sinteredbody 1 has a relative density within the above range, pores are hardlyformed in the titanium sintered body 1. Due to this, the inhibition oflight reflection by such pores can be suppressed.

The relative density of the titanium sintered body 1 is a dry densitymeasured in accordance with the test method of density of sintered metalmaterials specified in JIS Z 2501:2000.

Further, the Vickers hardness (HV) of the titanium sintered body 1 isnot particularly limited, but is preferably 300 or more, more preferably350 or more and 600 or less. The titanium sintered body 1 having such ahardness is less likely to be scratched or the like on the surface. Dueto this, the titanium sintered body 1 capable of suppressing thedeterioration of the sense of beauty by a scratch or the like even ifthe titanium sintered body 1 is used as a constituent material of, forexample, an ornament or the like is obtained.

The Vickers hardness (HV) of the titanium sintered body 1 is measured onthe surface of the titanium sintered body 1, and the measurement methodis in accordance with Vickers hardness test—Test method specified in JISZ 2244:2009. Incidentally, a test force applied by an indenter is set to9.8 N (1 kgf), and a test force duration is set to 15 seconds. Then, anaverage of the measurement results at 10 sites is determined as thesurface Vickers hardness.

Such a titanium sintered body 1 can be applied to various uses and isparticularly useful as a constituent material of an ornament, althoughthe use thereof is not particularly limited.

Ornament

Next, an embodiment of an ornament according to the invention will bedescribed.

Examples of the ornament according to the invention include externalcomponents for watches such as watch cases (case bodies, case backs,one-piece cases in which a case body and a case back are integrated,etc.), watch bands (including band clasps, band-bangle attachmentmechanisms, etc.), bezels (for example, rotatable bezels, etc.), crowns(for example, screw-lock crowns, etc.), buttons, glass frames, dialrings, etching plates, and packings, personal ornaments such as glasses(for example, glasses frames), tie clips, cuff buttons, rings,necklaces, bracelets, anklets, brooches, pendants, earrings, and piercedearrings, utensils such as spoons, forks, chopsticks, knives, butterknives, and corkscrews, lighters or lighter cases, sports goods such asgolf clubs, nameplates, panels, prize cups, and external components forapparatuses such as housings (for example, housings for cellular phones,smartphones, tablet terminals, mobile computers, music players, cameras,shavers, etc.). For any of these ornaments, excellent aestheticappearance is sometimes considered important. By using the titaniumsintered body 1 as a constituent material of these ornaments, excellentspecularity can be imparted to the surface of the ornaments. Accordingto this, ornaments capable of maintaining excellent aesthetic appearancefor a long period of time is obtained.

FIG. 3 is a perspective view showing a watch case to which theembodiment of the ornament according to the invention is applied. FIG. 4is a partial cross-sectional perspective view showing a bezel to whichthe embodiment of the ornament according to the invention is applied.

A watch case 11 shown in FIG. 3 includes a case main body 112 and a bandattachment section 114 for attaching a watch band provided protrudingfrom the case main body 112. Such a watch case 11 can form a containeralong with a glass plate (not shown) and a case back (not shown). Inthis container, a movement (not shown), a dial plate (not shown), etc.are housed. Therefore, this container protects the movement and the likefrom the external environment and also has a large influence on theaesthetic appearance of the watch.

A bezel 12 shown in FIG. 4 has an annular shape, and is attached to awatch case, and is rotatable with respect to the watch case as needed.When the bezel 12 is attached to a watch case, the bezel 12 is locatedoutside the watch case, and therefore has an influence on the aestheticappearance of the watch.

Further, such a watch case 11 and a bezel 12 are used in a state wherethey are attached to the human body, and therefore are likely to bescratched at all times. Due to this, by using the titanium sintered body1 as a constituent material of such an ornament, an ornament having highspecularity on the surface and also having excellent aestheticappearance is obtained. In addition, this specularity can be maintainedfor a long period of time.

Method for Producing Titanium Sintered Body

Next, a method for producing the titanium sintered body 1 will bedescribed.

The method for producing the titanium sintered body 1 includes [1] astep of obtaining a kneaded material by kneading a titanium alloy powderand an organic binder, [2] a step of obtaining a molded body by moldingthe kneaded material by a powder metallurgy method, [3] a step ofobtaining a degreased body by degreasing the molded body, [4] a step ofobtaining a sintered body by firing the degreased body, and [5] a stepof performing a hot isostatic pressing treatment (HIP treatment) for thesintered body. Hereinafter, the respective steps will be sequentiallydescribed.

[1] Kneading Step

First, a titanium simple substance powder or a titanium alloy powder(hereinafter simply referred to as “titanium alloy powder”) to serve asa starting material of the titanium sintered body 1 is kneaded alongwith an organic binder, whereby a kneaded material is obtained.

The average particle diameter of the titanium alloy powder is notparticularly limited, but is preferably 1 μm or more and 50 μm or less,more preferably 5 μm or more and 40 μm or less.

The titanium alloy powder may be a powder (a pre-alloy powder) composedonly of particles having a single alloy composition or may be a mixedpowder (a pre-mix powder) obtained by mixing a plurality of types ofparticles having different compositions from one another. In the case ofa pre-mix powder, an individual particle may be a particle containingonly one type of element or a particle containing a plurality ofelements as long as a compositional ratio as described above issatisfied as a whole pre-mix powder.

The content of the organic binder in the kneaded material isappropriately set according to the molding conditions, the shape to bemolded, or the like, but is preferably about 2 mass % or more and 20mass % or less, more preferably about 5 mass % or more and 10 mass % orless of the total amount of the kneaded material. By setting the contentof the organic binder within the above range, the kneaded material hasfavorable fluidity. According to this, the filling property of thekneaded material when performing molding is improved, and a sinteredbody having a shape closer to a final desired shape (near-net shape) isobtained.

Examples of the organic binder include polyolefins such as polyethylene,polypropylene, and ethylene-vinyl acetate copolymers, acrylic resinssuch as polymethyl methacrylate and polybutyl methacrylate, styrenicresins such as polystyrene, polyesters such as polyvinyl chloride,polyvinylidene chloride, polyamide, polyethylene terephthalate, andpolybutylene terephthalate, various resins such as polyether, polyvinylalcohol, polyvinylpyrrolidone, and copolymers thereof, and variousorganic binders such as various waxes, paraffins, higher fatty acids(such as stearic acid), higher alcohols, higher fatty acid esters, andhigher fatty acid amides. These can be used alone or by mixing two ormore types thereof.

In the kneaded material, a plasticizer may be added as needed. Examplesof the plasticizer include phthalate esters (such as DOP, DEP, and DBP),adipate esters, trimellitate esters, and sebacate esters. These can beused alone or by mixing two or more types thereof.

Further, in the kneaded material, other than the titanium alloy powder,the organic binder, and the plasticizer, for example, any of a varietyof additives such as a lubricant, an antioxidant, a degreasingaccelerator, and a surfactant can be added as needed.

The kneading conditions vary depending on the respective conditions suchas the alloy composition or the particle diameter of the titanium alloypowder to be used, the composition of the organic binder, and theblending amount thereof. However, for example, the kneading temperaturecan be set to about 50° C. or higher and 200° C. or lower, and thekneading time can be set to about 15 minutes or more and 210 minutes orless.

Further, the kneaded material is formed into a pellet (small particle)as needed. The particle diameter of the pellet is set to, for example,about 1 mm or more and 15 mm or less.

Incidentally, depending on the molding method described below, in placeof the kneaded material, a granulated powder may be produced.

[2] Molding Step

Subsequently, the kneaded material is molded, whereby a molded body isproduced.

The molding method is not particularly limited, and for example, any ofa variety of molding methods such as a powder compacting (compressionmolding) method, a metal injection molding (MIM) method, and anextrusion molding method can be used. Among these, from the viewpointthat a sintered body having a near-net shape can be produced, a metalinjection molding method is preferably used.

The molding conditions in the case of a powder compacting method arepreferably such that the molding pressure is about 200 MPa or more and1000 MPa or less (2 t/cm² or more and 10 t/cm² or less), which varydepending on the respective conditions such as the composition and theparticle diameter of the titanium alloy powder to be used, thecomposition of the organic binder, and the blending amount thereof.

The molding conditions in the case of the titanium alloy powder arepreferably such that the material temperature is about 80° C. or higherand 210° C. or lower, and the injection pressure is about 50 MPa or moreand 500 MPa or less (0.5 t/cm² or more and 5 t/cm² or less), which alsovary depending on the respective conditions.

The molding conditions in the case of an extrusion molding method arepreferably such that the material temperature is about 80° C. or higherand 210° C. or lower, and the extrusion pressure is about 50 MPa or moreand 500 MPa or less (0.5 t/cm² or more and 5 t/cm² or less), which alsovary depending on the respective conditions.

The thus obtained molded body is in a state where the organic binder isuniformly distributed in gaps between the particles of the titaniumalloy powder.

The shape and size of the molded body to be produced are determined inanticipation of shrinkage of the molded body in the subsequentdegreasing step and firing step.

Further, according to need, the molded body may be subjected tomachining processing such as grinding, polishing, or cutting. The moldedbody has a relatively low hardness and relatively high plasticity, andtherefore, the machining processing can be easily performed whilepreventing the molded body from losing its shape. According to suchmachining processing, the titanium sintered body 1 having highdimensional accuracy can be more easily obtained in the end.

[3] Degreasing Step

Subsequently, the thus obtained molded body is subjected to a degreasingtreatment (binder removal treatment), whereby a degreased body isobtained.

Specifically, the degreasing treatment is performed in such a mannerthat the organic binder is decomposed by heating the molded body,whereby at least part of the organic binder is removed from the moldedbody.

Examples of the degreasing treatment include a method of heating themolded body and a method of exposing the molded body to a gas capable ofdecomposing the binder.

In the case of using a method of heating the molded body, the conditionsfor heating the molded body are preferably such that the temperature isabout 100° C. or higher and 750° C. or lower and the time is about 0.1hours or more and 20 hours or less, and more preferably such that thetemperature is about 150° C. or higher and 600° C. or lower and the timeis about 0.5 hours or more and 15 hours or less, which slightly varydepending on the composition and the blending amount of the organicbinder. According to this, the degreasing of the molded body can benecessarily and sufficiently performed without sintering the moldedbody. As a result, it is possible to reliably prevent the organic bindercomponent from remaining inside the degreased body in a large amount.

The atmosphere when the molded body is heated is not particularlylimited, and an atmosphere of a reducing gas such as hydrogen, anatmosphere of an inert gas such as nitrogen or argon, an atmosphere ofan oxidative gas such as air, a reduced pressure atmosphere obtained byreducing the pressure of such an atmosphere, or the like can be used.

Examples of the gas capable of decomposing the binder include ozone gas.

Incidentally, by dividing this degreasing step into a plurality of stepsin which the degreasing conditions are different, and performing theplurality of steps, the organic binder in the molded body can be morerapidly decomposed and removed so that the organic binder does notremain in the molded body.

Further, according to need, the degreased body may be subjected tomachining processing such as grinding, polishing, or cutting. Thedegreased body has a relatively low hardness and relatively highplasticity, and therefore, the machining processing can be easilyperformed while preventing the degreased body from losing its shape.According to such machining processing, the titanium sintered body 1having high dimensional accuracy can be more easily obtained in the end.

(4) Firing Step

Subsequently, the obtained degreased body is fired in a firing furnace,whereby a sintered body is obtained. That is, diffusion occurs at theboundary surface between the particles of the titanium alloy powder,resulting in sintering. As a result, the titanium sintered body 1 isobtained.

The firing temperature varies depending on the composition, the particlediameter, and the like of the titanium alloy powder, but is set to, forexample, about 900° C. or higher and 1400° C. or lower, and preferablyset to about 1050° C. or higher and 1300° C. or lower.

The firing time is set to 0.2 hours or more and 20 hours or less, but ispreferably set to about 1 hour or more and 6 hours or less.

In the firing step, the firing temperature or the below-described firingatmosphere may be changed in the middle of the step.

The atmosphere when performing firing is not particularly limited,however, in consideration of prevention of significant oxidation of themetal powder, an atmosphere of a reducing gas such as hydrogen, anatmosphere of an inert gas such as argon, a reduced pressure atmosphereobtained by reducing the pressure of such an atmosphere, or the like ispreferably used.

In the case where the titanium sintered body 1 is produced from thetitanium alloy powder, depending on the firing conditions or the like,both α-phase 2 and β-phase 3 are sometimes formed. In particular, in thecase where the above-mentioned β-phase stabilizing element is containedin the titanium alloy powder, the β-phase 3 is more reliably formed.

On the other hand, by optimizing the respective production conditions,the ratio occupied by the α-phase 2 in the titanium sintered body 1,that is, the area ratio occupied by the α-phase 2 in the cross sectionof the titanium sintered body 1 can be adjusted. For example, when thefiring temperature is increased, the ratio of the β-phase 3 isincreased, and therefore, the firing temperature may be adjusted so thatthe ratio of the β-phase 3 falls within the desired range, and also thefiring time may be set in consideration of the increase in the size ofthe crystal structure caused by a too long firing time.

Therefore, for example, in the case where the titanium sintered body 1is produced using the titanium alloy powder which contains almost noβ-phase 3, depending on the composition of the titanium alloy powder,the ratio of the β-phase 3 is increased as the firing temperature isincreased, and therefore, the firing temperature is adjusted so that thearea ratio of the α-phase 2 falls within the above range, and also thefiring time is set so that insufficient sintering or excessive sinteringis not caused by adjusting the firing temperature.

Further, in the case where the average grain size of the α-phase 2 iswithin the above range, as the area ratio of the α-phase 2 is increased,the shape of the α-phase 2 tends to be close to an isotropic shape. Thisis considered to be because the probability that the grains of theα-phase 2 are located adjacent to each other is increased as the ratioof the β-phase 3 is decreased, and anisotropic grain growth is inhibitedby the grains of the α-phase 2 interfering with each other.

[5] HIP Step

The thus obtained sintered body may be further subjected to an HIPtreatment (hot isostatic pressing treatment) or the like. By doing this,the density of the sintered body is further increased, and thus, anornament having further excellent mechanical properties can be obtained.

As the conditions for the HIP treatment, for example, the temperature isset to 850° C. or higher and 1200° C. or lower, and the time is set toabout 1 hour or more and 10 hours or less.

Further, the pressure to be applied is preferably 50 MPa or more, morepreferably 100 MPa or more and 500 MPa or less.

In addition, the obtained sintered body may be further subjected to anannealing treatment, a solution heat treatment, an aging treatment, ahot working treatment, a cold working treatment, or the like as needed.

The obtained titanium sintered body 1 may be subjected to a polishingtreatment as needed. The polishing treatment is not particularlylimited, however, examples thereof include electrolytic polishing,buffing, dry polishing, chemical polishing, barrel polishing, and sandblasting. By performing such a polishing treatment, metallic luster isfurther given to the surface of the titanium sintered body 1, and thespecularity can be increased.

Hereinabove, the titanium sintered body and the ornament according tothe invention have been described with reference to preferredembodiments, however, the invention is not limited thereto.

For example, the use of the titanium sintered body is not limited to theornament, and may be various structural components and the like.Examples of the structural components include components for transportmachinery such as components for automobiles, components for bicycles,components for railroad cars, components for ships, components forairplanes, and components for space transport machinery (such asrockets), components for electronic devices such as components forpersonal computers and components for cellular phone terminals,components for electrical devices such as refrigerators, washingmachines, and cooling and heating machines, components for machines suchas machine tools and semiconductor production devices, components forplants such as atomic power plants, thermal power plants, hydroelectricpower plants, oil refinery plants, and chemical complexes, medicaldevices such as surgical instruments, artificial bones, jointprostheses, artificial teeth, artificial dental roots, and orthodonticcomponents.

The titanium sintered body has high biocompatibility, and therefore isparticularly useful as an artificial bone and a dental metal component.Among these, the dental metal component is not particularly limited aslong as it is a metal component which is temporarily or semipermanentlyretained in the mouth, and examples thereof include metal frames such asan inlay, a crown, a bridge, a metal base, a denture, an implant, anabutment, a fixture, and a screw.

EXAMPLES

Next, specific examples of the invention will be described.

1. Production of Titanium Sintered Body Example 1

(1) First, a Ti-6Al-4V alloy powder having an average particle diameterof 23 μm produced by gas atomization method was prepared.

Subsequently, a mixture (organic binder) of polypropylene and a wax wasprepared and weighed so that the mass ratio of the starting materialpowder to the organic binder was 9:1, whereby a composition forproducing a titanium sintered body was obtained.

Subsequently, the obtained composition for producing a titanium sinteredbody was kneaded using a kneader, whereby a compound was obtained. Then,the compound was processed into pellets.

(2) Subsequently, molding was performed under the following moldingconditions using the obtained pellets, whereby a molded body wasproduced.

Molding Conditions

Molding method: metal injection molding method

Material temperature: 150° C.

Injection pressure: 11 MPa (110 kgf/cm²)

(3) Subsequently, the obtained molded body was subjected to a degreasingtreatment under the following degreasing conditions, whereby a degreasedbody was obtained.

Degreasing Conditions

Degreasing temperature: 520° C.

Degreasing time: 5 hours

Degreasing atmosphere: nitrogen gas atmosphere

(4) Subsequently, the obtained degreased body was fired under thefollowing firing conditions, whereby a sintered body was produced.

Firing Conditions

Firing temperature: 1100° C.

Firing time: 5 hours

Firing atmosphere: argon gas atmosphere

Pressure in atmosphere: atmospheric pressure (100 kPa)

(5) Subsequently, the obtained sintered body was subjected to an HIPtreatment under the following treatment conditions, whereby a titaniumsintered body having the shape of a rod with a diameter of 5 mm and alength of 100 mm was obtained.

HIP Treatment Conditions

Treatment temperature: 900° C.

Treatment time: 3 hours

Treatment pressure: 1480 kgf/cm² (145 MPa)

(6) Subsequently, the obtained titanium sintered body was cut and thecut surface was polished by a buffing treatment.

Subsequently, the polished surface was observed by an electronmicroscope, and the average grain size of the α-phase, the area ratiosoccupied by the α-phase and the β-phase, and the average aspect ratio ofthe α-phase were obtained, respectively. The results are shown in Table1.

Examples 2 to 6

Titanium sintered bodies were obtained in the same manner as in Example1 except that the production conditions were changed so that the averagegrain size of the α-phase, the area ratios occupied by the α-phase andthe β-phase, and the average aspect ratio of the α-phase were as shownin Table 1, respectively.

Comparative Examples 1 to 3

Titanium sintered bodies were obtained in the same manner as in Example1 except that the production conditions were changed so that the averagegrain size of the α-phase, the area ratios occupied by the α-phase andthe β-phase, and the average aspect ratio of the α-phase were as shownin Table 1, respectively.

Reference Example 1

First, a Ti-6Al-4V alloy ingot material was prepared.

Subsequently, the prepared ingot material was cut and the cut surfacewas polished by a buffing treatment.

Subsequently, the polished surface was observed by an electronmicroscope, and the average grain size of the α-phase, the area ratiosoccupied by the α-phase and the β-phase, and the average aspect ratio ofthe α-phase were obtained, respectively. The results are shown in Table1.

Example 7

A titanium sintered body was obtained in the same manner as in Example 1except that a Ti-3Al-2.5V alloy powder having an average particlediameter of 23 μm was used in place of the Ti-6Al-4V alloy powder.

Then, the obtained titanium sintered body was cut and the cut surfacewas polished by a buffing treatment.

Subsequently, the polished surface was observed by an electronmicroscope, and the average grain size of the α-phase, the area ratiosoccupied by the α-phase and the β-phase, and the average aspect ratio ofthe α-phase were obtained, respectively. The results are shown in Table2.

Examples 8 to 12

Titanium sintered bodies were obtained in the same manner as in Example7 except that the production conditions were changed so that the averagegrain size of the α-phase, the area ratios occupied by the α-phase andthe β-phase, and the average aspect ratio of the α-phase were as shownin Table 2, respectively.

Comparative Examples 4 to 6

Titanium sintered bodies were obtained in the same manner as in Example7 except that the production conditions were changed so that the averagegrain size of the α-phase, the area ratios occupied by the α-phase andthe β-phase, and the average aspect ratio of the α-phase were as shownin Table 2, respectively.

Reference Example 2

First, a Ti-3Al-2.5V alloy ingot material was prepared.

Subsequently, the prepared ingot material was cut and the cut surfacewas polished by a buffing treatment.

Subsequently, the polished surface was observed by an electronmicroscope, and the average grain size of the α-phase, the area ratiosoccupied by the α-phase and the β-phase, and the average aspect ratio ofthe α-phase were obtained, respectively. The results are shown in Table2.

Example 13

A titanium sintered body was obtained in the same manner as in Example 1except that a Ti-6Al-7Nb alloy powder having an average particlediameter of 25 μm was used in place of the Ti-6Al-4V alloy powder.

Then, the obtained titanium sintered body was cut and the cut surfacewas polished by a buffing treatment.

Subsequently, the polished surface was observed by an electronmicroscope, and the average grain size of the α-phase, the area ratiosoccupied by the α-phase and the β-phase, and the average aspect ratio ofthe α-phase were obtained, respectively. The results are shown in Table3.

Examples 14 to 18

Titanium sintered bodies were obtained in the same manner as in Example13 except that the production conditions were changed so that theaverage grain size of the α-phase, the area ratios occupied by theα-phase and the β-phase, and the average aspect ratio of the α-phasewere as shown in Table 3, respectively.

Comparative Examples 7 to 9

Titanium sintered bodies were obtained in the same manner as in Example13 except that the production conditions were changed so that theaverage grain size of the α-phase, the area ratios occupied by theα-phase and the β-phase, and the average aspect ratio of the α-phasewere as shown in Table 3, respectively.

Reference Example 3

First, a Ti-6Al-7Nb alloy ingot material was prepared.

Subsequently, the prepared ingot material was cut and the cut surfacewas polished by a buffing treatment.

Subsequently, the polished surface was observed by an electronmicroscope, and the average grain size of the α-phase, the area ratiosoccupied by the α-phase and the β-phase, and the average aspect ratio ofthe α-phase were obtained, respectively. The results are shown in Table3.

2. Evaluation of Titanium Sintered Body 2.1. Specularity

First, with respect to each of the titanium sintered bodies and titaniumingot materials of the respective Examples, Comparative Examples, andReference Examples, the polished surface was visually observed. Then,the specularity of the polished surface was evaluated according to thefollowing evaluation criteria. The evaluation results are shown inTables 1 to 3.

Evaluation Criteria for Specularity of Polished Surface

A: The specularity of the polished surface is very high (the aestheticappearance is particularly good).

B: The specularity of the polished surface is slightly high (theaesthetic appearance is slightly good).

C: The specularity of the polished surface is slightly low (theaesthetic appearance is slightly poor).

D: The specularity of the polished surface is very low (the aestheticappearance is poor).

2.2. Relative Density

Subsequently, with respect to each of the titanium sintered bodies andtitanium ingot materials of the respective Examples, ComparativeExamples, and Reference Examples, the relative density was calculated inaccordance with the method specified in JIS Z 2501:2000. The calculationresults are shown in Tables 1 to 3.

2.3. Vickers Hardness

Subsequently, with respect to the polished surface of each of thetitanium sintered bodies and titanium ingot materials of the respectiveExamples, Comparative Examples, and Reference Examples, the Vickershardness was measured in accordance with the method specified in JIS Z2244:2009. The measurement results are shown in Tables 1 to 3.

2.4. Crystal Structure Analysis by X-ray Diffractometry

Subsequently, with respect to the titanium sintered body of Example 1, acrystal structure analysis was performed by X-ray diffractometry underthe following measurement conditions.

Measurement Conditions for Crystal Structure Analysis by X-rayDiffractometry

X-ray source: Cu-Kα radiation

Tube voltage: 30 kV

Tube current: 20 mA

The obtained X-ray diffraction spectrum is shown in FIG. 5.

As apparent from FIG. 5, it was found that the X-ray diffractionspectrum obtained for the titanium sintered body of Example 1 includes apeak reflection intensity by the α-phase (α-Ti) and a peak reflectionintensity by the β-phase (β-Ti). Then, the value of the peak reflectionintensity by the α-Ti in the plane orientation (100) located at 2θ ofabout 35.3° was used as the standard, and the ratio (peak ratio) of thevalue of the peak reflection intensity by the β-Ti in the planeorientation (110) located at 2θ of about 39.5° to the standard wascalculated. In addition, the same calculation was also performed foreach of the titanium sintered bodies and titanium ingot materials ofExamples 2 to 18, Comparative Examples 1 to 9, and Reference Examples 1to 3. The calculation results of the peak ratio are shown in Table 1 to3.

2.5. Wear Resistance

Subsequently, with respect to each of the titanium sintered bodies ofthe respective Examples and Comparative Examples and each of thetitanium ingot materials and the like of the respective ReferenceExamples, the wear resistance of the surface thereof was evaluated.Specifically, first, the surface of each of the titanium sintered bodiesand the titanium ingot materials was polished by a buffing treatment.Subsequently, for the polished surface, a wear resistance test wasperformed in accordance with Testing method for wear resistance of fineceramics by ball-on-disc method specified in JIS R 1613 (2010), and awear amount of a disk-shaped test piece was measured. The measurementconditions were as follows.

Measurement Conditions for Specific Wear Amount

Material of spherical test piece: high carbon chromium bearing steel(SUJ2)

Size of spherical test piece: diameter: 6 mm

Material of disk-shaped test piece: each of titanium sintered bodies ofrespective Examples and Comparative Examples and each of titanium ingotmaterials of respective Reference Examples

Size of disk-shaped test piece: diameter: 35 mm, thickness: 5 mm

Magnitude of load: 10 N

Sliding rate: 0.1 m/s

Sliding circle diameter: 30 mm

Sliding distance: 50 m

Then, the wear amount obtained for the titanium ingot material ofReference Example 1 was taken as 1, and the relative value of the wearamount obtained for each of the titanium sintered bodies of therespective Examples and Comparative Examples shown in Table 1 wascalculated.

Similarly, the wear amount obtained for the titanium ingot material ofReference Example 2 was taken as 1, and the relative value of the wearamount obtained for each of the titanium sintered bodies of therespective Examples and Comparative Examples shown in Table 2 wascalculated.

Further similarly, the wear amount obtained for the titanium ingotmaterial of Reference Example 3 was taken as 1, and the relative valueof the wear amount obtained for each of the titanium sintered bodies ofthe respective Examples and Comparative Examples shown in Table 3 wascalculated.

Then, the calculated relative value was evaluated according to thefollowing evaluation criteria. The evaluation results are shown inTables 1 to 3.

Evaluation Criteria for Wear Amount

A: The wear amount is very small (the relative value is less than 0.5).

B: The wear amount is small (the relative value is 0.5 or more and lessthan 0.75).

C: The wear amount is slightly small (the relative value is 0.75 or moreand less than 1).

D: The wear amount is slightly large (the relative value is 1 or moreand less than 1.25).

E: The wear amount is large (the relative value is 1.25 or more and lessthan 1.5).

F: The wear amount is very large (the relative value is more than 1.5).

2.6. Tensile Strength

Subsequently, with respect to each of the titanium sintered bodies ofthe respective Examples and Comparative Examples and each of thetitanium ingot materials and the like of the respective ReferenceExamples, the tensile strength was measured. The measurement of thetensile strength was performed in accordance with the metal materialtensile test method specified in JIS Z 2241 (2011).

Then, the tensile strength obtained for the titanium ingot material ofReference Example 1 was taken as 1, and the relative value of thetensile strength obtained for each of the titanium sintered bodies ofthe respective Examples and Comparative Examples shown in Table 1 wascalculated.

Similarly, the tensile strength obtained for the titanium ingot materialof Reference Example 2 was taken as 1, and the relative value of thetensile strength obtained for each of the titanium sintered bodies ofthe respective Examples and Comparative Examples shown in Table 2 wascalculated.

Further similarly, the tensile strength obtained for the titanium ingotmaterial of Reference Example 3 was taken as 1, and the relative valueof the tensile strength obtained for each of the titanium sinteredbodies of the respective Examples and Comparative Examples shown inTable 3 was calculated.

Then, the obtained relative value was evaluated according to thefollowing evaluation criteria. The evaluation results are shown inTables 1 to 3. As for the tensile strength, other than theabove-mentioned test specimens, also an SUS316L sintered body, a castmaterial and a sintered body of ASTM F75 (a CO-28% Cr-6% Mo alloy), andan α-Ti sintered body were evaluated as Reference Examples a to d (Table1). Further, with respect to Reference Example d, the same evaluation asin the above-mentioned 2.1. to 2.3. and 2.5. was performed in additionto 2.6.

Evaluation Criteria for Tensile Strength

A: The tensile strength is very large (the relative value is 1.09 ormore).

B: The tensile strength is large (the relative value is 1.06 or more andless than 1.09).

C: The tensile strength is slightly large (the relative value is 1.3 ormore and less than 1.06).

D: The tensile strength is slightly small (the relative value is 1 ormore and less than 1.03).

E: The tensile strength is small (the relative value is 0.97 or more andless than 1).

F: The tensile strength is very small (the relative value is less than0.97).

2.7. Nominal Strain at Break (Elongation at Break)

Subsequently, with respect to each of the titanium sintered bodies ofthe respective Examples and Comparative Examples and each of thetitanium ingot materials and the like of the respective ReferenceExamples, the elongation at break was measured. The measurement of theelongation at break was performed in accordance with the metal materialtensile test method specified in JIS Z 2241 (2011).

Then, the obtained elongation at break was evaluated according to thefollowing evaluation criteria. The evaluation results are shown inTables 1 to 3. As for the elongation at break, other than theabove-mentioned test specimens, also an SUS316L sintered body, a castmaterial and a sintered body of ASTM F75 (a CO-28% Cr-6% Mo alloy), andan α-Ti sintered body were evaluated as Reference Examples a to d (Table1).

Evaluation Criteria for Elongation at Break

A: The elongation at break is very large (0.15 or more).

B: The elongation at break is large (0.125 or more and less than 0.15).

C: The elongation at break is slightly large (0.10 or more and less than0.125).

D: The elongation at break is slightly small (0.075 or more and lessthan 0.10).

E: The elongation at break is small (0.050 or more and less than 0.075).

F: The elongation at break is very small (less than 0.050).

2.8. Cytotoxicity Test

Subsequently, with respect to a test specimen composed of each of thetitanium sintered bodies of the respective Examples and ComparativeExamples and each of the titanium ingot materials and the like of therespective Reference Examples, a cytotoxicity test was performed. Thecytotoxicity test was performed in accordance with the cytotoxicity testspecified in ISO 10993-5:2009. Specifically, by a colony formationmethod using a direct contact method, an average of the number ofcolonies in a control group is taken as 100%, and the ratio of thenumber of colonies of cells directly inoculated onto the test specimento the number of colonies in the control group (colony formation ratio(%)) was obtained. The test conditions were as follows.

Cell line: V97 cell line

Culture medium: MEM10 medium

Negative control material (negative control): high-density polyethylenefilm

Positive control material (positive control): 0.1% zincdiethyldithiocarbamate-containing polyurethane film

Control group (control): the number of colonies of cells directlyinoculated into the culture medium

Subsequently, the obtained colony formation ratio was classifiedaccording to the following evaluation criteria, whereby the cytotoxicityof each test specimen was evaluated. The evaluation results are shown inTables 1 to 3. As for the cytotoxicity test, other than theabove-mentioned test specimens, also an SUS316L sintered body, asintered body of ASTM F75 (a CO-28% Cr-6% Mo alloy), and an α-Tisintered body were evaluated as Reference Examples a, c, and d (Table1).

Evaluation Criteria for Cytotoxicity

A: The colony formation ratio is 90% or more.

B: The colony formation ratio is 80% or more and less than 90%.

C: The colony formation ratio is less than 80%.

TABLE 1 Structure of titanium sintered body α-phase X-ray Evaluationresults Aver- β- diffrac- Rela- Elon- Cyto- Produc- Crystal age phasetion tive Vickers Wear gation tox- tion struc- grain Area Aspect Areapeak Specu- den- hard- resis- Tensile at icity method Composition turesize ratio ratio ratio ratio larity sity ness tance strength break test— — — μm % — % % — % — — — — — Example 1 Sintered Ti—6Al—4V α + β 15 821.8 18 28 A 99.8 380 A A B B body Example 2 Sintered Ti—6Al—4V α + β 1288 1.6 12 22 A 99.7 395 A A B B body Example 3 Sintered Ti—6Al—4V α + β24 78 2.2 22 32 A 99.5 360 B B B B body Example 4 Sintered Ti—6Al—4V α +β 8 92 1.3 8 18 A 99.6 410 B B B B body Example 5 Sintered Ti—6Al—4V α +β 28 72 2.5 28 38 B 99.3 340 C C B B body Example 6 Sintered Ti—6Al—4Vα + β 5 85 1.9 15 25 B 99.1 425 B C B B body Comp. Sintered Ti—6Al—4Vα + β 2 77 1.4 23 33 C 98.5 400 D D C B Ex. 1 body Comp. SinteredTi—6Al—4V α + β 35 71 4.5 29 51 D 96.4 230 D E C B Ex. 2 body Comp.Sintered Ti—6Al—4V α + β 27 66 7.6 34 63 D 97.2 240 D E C B Ex. 3 bodyRef. Ingot Ti—6Al—4V α + β 4 90 3.1 10 20 B 99.8 350 D C C B Ex. 1material Ref. Sintered SUS316L — — — — — — — — — — F A A Ex. a body Ref.Cast F75 — — — — — — — — — — C C — Ex. b material Ref. Sintered F75 — —— — — — — — — — D D B Ex. c body Ref. Sintered α-Ti α 5 99.9 2.4 0.1 — D96.5 210 E E E B Ex. d body Negative control material — A Positivecontrol material — C

TABLE 2 Structure of titanium sintered body α-phase X-ray Evaluationresults Aver- β- diffrac- Rela- Elon- Cyto- Produc- Crystal age phasetion tive Vickers Wear gation tox- tion struc- grain Area Aspect Areapeak Specu- den- hard- resis- Tensile at icity method Composition turesize ratio ratio ratio ratio larity sity ness tance strength break test— — — μm % — % % — % — — — — — Example 7 Sintered Ti—3Al—2.5V α + β 1883 2.0 17 27 A 99.8 370 A A B B body Example 8 Sintered Ti—3Al—2.5V α +β 11 89 1.7 11 21 A 99.7 380 A A B B body Example 9 Sintered Ti—3Al—2.5Vα + β 25 79 2.3 21 31 A 99.5 350 A B B B body Example 10 SinteredTi—3Al—2.5V α + β 9 93 1.4 7 17 A 99.4 400 B B B B body Example 11Sintered Ti—3Al—2.5V α + β 27 73 2.6 27 37 B 99.2 330 C C B B bodyExample 12 Sintered Ti—3Al—2.5V α + β 6 84 2.3 16 26 B 99.1 415 B C B Bbody Comp. Sintered Ti—3Al—2.5V α + β 2 75 1.7 25 35 C 98.6 390 D D C BEx. 4 body Comp. Sintered Ti—3Al—2.5V α + β 36 72 4.8 28 52 D 96.5 220 DE C B Ex. 5 body Comp. Sintered Ti—3Al—2.5V α + β 28 68 8.2 32 64 D 97.8230 D E C B Ex. 6 body Ref. Ingot Ti—3Al—2.5V α + β 3 91 3.2 9 19 B 99.7340 D C C B Ex. 2 material

TABLE 3 Structure of titanium sintered body α-phase X-ray Evaluationresults Aver- β- diffrac- Rela- Elon- Cyto- Produc- Crystal age phasetion tive Vickers Wear gation tox- tion struc- grain Area Aspect Areapeak Specu- den- hard- resis- Tensile at icity method Composition turesize ratio ratio ratio ratio larity sity ness tance strength break test— — — μm % — % % — % — — — — — Example 13 Sintered Ti—6Al—7Nb α + β 1390 1.9 10 20 A 99.7 390 A A B A body Example 14 Sintered Ti—6Al—7Nb α +β 9 96 1.5 4 14 A 99.8 400 A A B A body Example 15 Sintered Ti—6Al—7Nbα + β 22 80 2.1 20 30 A 99.3 360 B B B A body Example 16 SinteredTi—6Al—7Nb α + β 7 98 1.3 2 12 A 99.5 420 A B B A body Example 17Sintered Ti—6Al—7Nb α + β 25 78 2.2 22 32 B 99.0 340 C C B A bodyExample 18 Sintered Ti—6Al—7Nb α + β 5 92 1.8 8 18 B 99.2 440 B C B Abody Comp. Sintered Ti—6Al—7Nb α + β 2 76 1.6 24 34 C 98.4 410 D D C AEx. 7 body Comp. Sintered Ti—6Al—7Nb α + β 37 71 4.9 29 53 D 96.6 210 DE C A Ex. 8 body Comp. Sintered Ti—6Al—7Nb α + β 29 65 7.8 35 65 D 97.6220 D E C A Ex. 9 body Ref. Ingot Ti—6Al—7Nb α + β 4 93 3.5 7 17 B 99.5330 D B C A Ex. 3 material

As apparent from Tables 1 to 3, it was confirmed that the titaniumsintered bodies of the respective Examples have high specularity on thepolished surface. Further, since the relative density and the Vickershardness are also high, it was confirmed that the titanium sinteredbodies of the respective Examples can maintain high specularity for along period of time.

It was also confirmed that the characteristics such as specularity,density, and hardness of the titanium sintered bodies of the respectiveExamples are equivalent to or higher than those of the titanium ingotmaterials. Therefore, according to the invention, a titanium sinteredbody having excellent characteristics can be obtained while takingadvantage of the characteristic of near-net shape.

An electron microscopic image of a cross section of the titaniumsintered body of Comparative Example 2 is shown in FIG. 6. From FIG. 6,it is confirmed that in the titanium sintered body of ComparativeExample 2, the α-phase has an elongated shape, that is, a shape withlarge anisotropy.

Further, an electron microscopic image of a cross section of thetitanium ingot material of Reference Example 1 is shown in FIG. 7. FromFIG. 7, it is confirmed that in the titanium ingot material of ReferenceExample 1, although the grain size of the α-phase is relatively small,the α-phase has a shape with large anisotropy.

What is claimed is:
 1. A titanium sintered body, wherein the titanium sintered body contains an α-phase and a β-phase as crystal structures, the average grain size of the α-phase in a cross section is 3 μm or more and 30 μm or less, and an area ratio occupied by the α-phase in a cross section is 70% or more and 99.8% or less.
 2. The titanium sintered body according to claim 1, wherein the average aspect ratio of the α-phase in a cross section is 1 or more and 3 or less.
 3. The titanium sintered body according to claim 1, wherein in an X-ray diffraction spectrum obtained by X-ray diffractometry, the value of a peak reflection intensity by the plane orientation (110) of the β-phase is 3% or more and 60% or less of the value of a peak reflection intensity by the plane orientation (100) of the α-phase.
 4. The titanium sintered body according to claim 1, wherein the titanium sintered body contains titanium as a main component, and also contains an α-phase stabilizing element and a β-phase stabilizing element.
 5. The titanium sintered body according to claim 1, wherein the titanium sintered body has a relative density of 99% or more.
 6. An ornament comprising the titanium sintered body according to claim
 1. 7. An ornament comprising the titanium sintered body according to claim
 2. 8. An ornament comprising the titanium sintered body according to claim
 3. 9. An ornament comprising the titanium sintered body according to claim
 4. 10. An ornament comprising the titanium sintered body according to claim
 5. 